An inverted V for 15m/21MHz

Part #16 of the "Roger Writes" series - February 2025

Background

The RSGB are/were holding a fun challenge on the 15m band in March 2025 for North and South Wales (Regions 6 and 7 competition in March 2025). One of the main rules is "You must use a homebrew wire antenna, which is resonant on 21MHz".
So it was time to work out some simple designs.

Simple inverted-V

One of the simplest antennas is an inverted-V, the calculation I remember is: total length=468/MHz
This gives the total length of both legs (in feet) for 21.225MHz (centre of the band) as 22feet (so 6.72meters).
I modelled it in 4nec2, here is the model. It comes out a little longer at 6.9 meters.

Making it

I used a centre tee:

But you can just solder the coax directly to the legs, or use a connector block, no special circuitry or balun is required.

Always cut the wire long, and fold them back at the ends.

The target length is from the centre of the coax feed to the end of the loop.

If the SWR minimum is at a low frequency then the antenna is too long, so you need to shorten the wires, if the frequency is too high, it's too short, so lengthen them. You can estimate how much to change it by, by calculating the error as a percentage of the frequency, and modifying the wire length by that percentage. The null point was at about 20.2MHz, so about 5% low, with 3.45m 5% is about 15cm, so I tied them about 15cm shorter. Once you've finished tweaking, best to double check both wires are the same length.

I ended up with 3.30m/130" each side, the dip point is a little low (at 21.1MHz), so I could shorten the antenna a little, and vary the angle to improve the SWR, but it's good enough as is:
1:1.218 at 21.0MHz

1:1.476 at 21.45MHz


I was testing this on a wooden pole, with the ends tied to nylon cord going to whatever was handy. So the centre was about 2.5m from the ground, and the ends were about 1.5m. Raising it higher is likely to change these results, but the SWR is flat enough that even with it raised up, the whole band will probably be under 2.5:1.

You should use a balun to limit the coax interaction with the antenna, for 15m about 6 coils of 15cm/6" in diameter should be enough.

The Roger Writes series

I research / dabble with lots of things, and figured that if I write my notes here, I can quickly reference them, also, sometimes, they are useful to others!
Here is what I have so far:




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